Hydrogen-induced cracking (HIC) is encountered by oil and gas pipelines and related installations with sour environments having high hydrogen sulfide (H2S) concentrations. These defects are attributable to atomic hydrogen produced by sour corrosion that enters the bulk of the steel. The atomic hydrogen reacts and recombines to form high pressure molecular hydrogen cavities at the interface of nonmetallic spaces residing in the microstructure. HIC tends to propagate in a plane parallel to the pipe wall as shown in FIG. 1, which shows examples of cracks induced by HIC. Fracture toughness (FT) tests are standardized mechanical tests design to measure the resistance of a material to crack growth. In FT tests, a pre-cracked test specimen is loaded under a controlled displacement rate while measuring the resulting force. A force-displacement curve is used to calculate FT parameters such as a plain strain stress intensity factor (K) and a J-integral (J).
When carrying out fracture toughness (FT) tests to characterize the ability of the material to resist crack propagation, the dimensions and orientation of the FT specimen are critical. The dimensions of a rectangular forged/rolled plate sample are defined as the longitudinal (L) which is parallel to the plate rolling/forging direction, the transverse dimension (T) and the short transverse or thickness dimension (S). A schematic model of a sample illustrating these planes is shown in FIG. 2. The first letter denotes the direction normal to the crack plane (which coincides with the direction of the principal tensile stress for Model I fracture), while the second letter denotes the direction of crack extension.
The directions of interest for HIC crack propagation and more generally, stepwise cracking, are the S-T or S-L directions shown in FIG. 2, which are the directions in which parallel in-plane cracks occur. It has proven to be difficult to measure fracture toughness (FT) properties for thin or relatively thin pipelines (10-30 mm wall thickness) in these directions. This is particularly problematic, as often the FT values in the SL and ST direction are not equal to the FT values in the other directions (e.g. TL, LT), so that measurements taken in the other directions cannot be used as a reliable estimate for the FT values in the SL and ST directions.
The ASTM (American Society for Testing and Materials) 1820 fracture toughness test standard requires use of specific specimens, of either a single edge bending (SEB) or a compact tension (CT) type. However, such specimens are not suited for FT measurements in S-T and S-L directions because there is not enough material in the thickness direction to extract a full SEB or CT specimens. For example, to machine a typical SEB specimen of 10 millimeter thickness requires a minimum plate thickness of about 90 millimeters, which is well above common pipe thicknesses of pipe equipment used in the oil and gas industries.
While in-plane FT data is not required to design against fracture of metallic structures, it becomes of high interest when the equipment of interest may develop in-plane cracks such as HIC. Such data can help at the material selection stage to discriminate between different types of steel, the quality of metal provided by different manufacturers, and also can enable prediction of crack growth rate and their impact on the residual integrity of the equipment during their service life.
What is therefore need is a methodology to enable FT tests for in-plane fractures that can produce valid measurements (i.e., compliant with the Standard) of the in-plane fracture toughness of metallic plates. It is with respect to these and other considerations that the disclosure made herein is presented.